This present invention relates generally to telecommunication systems and more particularly to loading carriers in a multi-carrier system.
In today""s modern world, businesses and residential users are demanding faster network access to the Internet. The high demand for faster network access is putting pressure on vendors and service providers to choose network transmission technologies that will satisfy the emerging demand. The choice of network transmission technologies is critical since it may affect service, cost, and ultimately vendor/service provider success.
Many of the vendors and service providers have chosen to pursue digital subscriber line (DSL) technology and more specifically asymmetrical DSL (ADSL) for providing fast Internet access to business and residential users. ADSL often provides high-speed data transmission over standard phone lines while maintaining voice traffic on the same lines. ADSL is seen as a cost-effective alternative to other network transmission technologies. High speed data transmission, however, may be best achieved by utilizing ADSL modems offering automatic rate adaptation, permitting the maximum data rate based on achievable data-carrying capacity of any given line. Presently, American National Standards Institute (ANSI) T1.413 standard discrete multitone (DMT)-based ADSL modems provide for rate adaptation. Although non-DMT based ADSL modems may be configured to handle variable data rates, the configuration may be complicated and costly.
ADSL technology exploits the relatively high bandwidth of copper loops by converting twisted-pair copper telephone wires into paths for multimedia, data communications, and Internet access. Typically, ADSL supports 1.544 to 6 Mbps transmission downstream and 640 kb/s upstream. ADSL service may be provided by connecting a pair of modems, one often located in the telephone company""s central office (CO) and the other located at the customer premises, over a standard telephone line.
ADSL modems offering automatic rate adaptation may permit the maximum data rate based on achievable data-carrying capacity of any given line. Rate adaptation is the ability to automatically correlate the connected data rate to distance extremes (e.g., distance between the CO and the customer premise), interference, and noise to provide the maximum date rate for any line conditions. Noise may include crosstalk from other services, near end echo, analog to digital conversion quantization, hardware noise, jitter, and intersymbol interference.
An ADSL modem utilizing ANSI appointed DMT as the modulation scheme segments the frequency spectrum on a copper line into 256 channels. Each 4 kHz channel is capable of carrying up to 15 data bits and a minimum of 2 data bits due to ANSI T1.413 standard. During channel analysis, a wide-band test signal sent over the 256 channels is transmitted from the ADSL terminal unit at the CO (ATU-C) to an ADSL remote terminal unit (ATU-R) at the customer premises. The ATU-R measures and updates the noise content of each of the channels received and then determines whether a channel has sufficient quality to be used for further transmission. Depending on the quality, the ATU-R may instruct the ATU-C how much data this channel should carry relative to the other channels that are used. Often, this procedure maximizes performance and minimizes error probability at any data specific rate. For instance, with a DMT modem, bit distribution may avoid noise by not loading bits onto channels that are corrupted by AM radio interference. The DMT modem may also lower bit distribution at the frequencies where notching occurs.
However, due to ADSL ANSI T1.413, standard requirements the minimum amount of bits a channel may support referred to as bit loading, are 2 bits often rendering bit loading methods sub-optimal. Often times during channel analysis one or more channels may be capable of carrying one bit and as a result, according to ANSI T1.413, is inactivated. Many bit loading processes do not make a concerted effort to re-activate these inactivated channels perhaps leading to reduced throughput.
Thus there is a need for effectively bit loading channels with a concerted effort to re-activate channels and to increase available throughput of both upstream and downstream channels.
The system and method for providing bit loading enhancement preferably re-activates unloaded channels to increase available throughput of both upstream and downstream channels. The method attempts to increase available throughput while maximizing power savings by determining an extended bit capacity and optimizing the average signal to noise margin.
In accordance with one aspect, a method for determining an extended bit capacity for a set of carriers includes a first portion of carriers that are loaded and a second portion of carriers that are initially unloaded. The method includes the step of determining a first power requirement for allocating one bit onto each of two carriers of the first portion and determining a second power requirement for allocating two bits onto one carrier of the second portion. The method then includes allocating two bits onto the one carrier of the second portion in accordance with the first power requirement and the second power requirement. If the first power requirement is greater than the second power requirement, then two bits are allocated onto the one carrier of the second portion. Otherwise, one bit is allocated onto one carrier of the first portion.
In accordance with another aspect, a method for optimizing the average signal to noise margin for a set of carriers includes a first portion of the carriers that are loaded and a second portion of the carriers that are initially unloaded. The method includes the step of determining a power saving resulting from removing one bit from each of two carriers of the first portion and determining a power requirement to load two bits onto one carrier of the second portion. The method then includes loading two bits onto one carrier of the second portion until the power requirement is greater than or equal to the power saving.
The system and method for bit loading enhancement makes a concerted effort to re-activate unloaded channels perhaps leading to increased throughput. The system and method attempts to bit load initially unloaded carriers to increase the frame size transmitted over a set of channels. To accomplish increased throughput, power is obtained from carefully chosen loaded carriers and applied to initially unloaded carriers attempting to reactivate the initially unloaded carriers. As a result, the set of carriers may have a net gain of data carrying bits without increasing the nominal power spectral density of the transmitted discrete multitone signal within the pass band. Thus, the system and method may attempt to re-activate unloaded channels to increase available throughput for both upstream and downstream channels.